This invention relates to master cylinders and more particularly to master cylinders especially suited for use in a master/slave hydraulic control system.
Master cylinders are in common usage such, for example, as in combination with a slave cylinder to provide the actuating mechanism for a mechanical clutch of a motor vehicle.
A typical master cylinder assembly includes a casing structure defining a cylindrical bore and a piston slidably mounted in the bore. Pressurized hydraulic fluid is discharged from the cylindrical bore for delivery to the slave cylinder in response to stroking reciprocal movement of the piston in the bore. Master cylinder assemblies were originally formed primarily of metallic materials but more recently the cylinder bodies have been formed of a plastic material to facilitate the manufacturing process and reduce the cost of the assembly.
Forming the cylindrical body of a plastic material has the disadvantage that either the bore must be tapered to provide a certain amount of draft to facilitate unmolding, or complicated and expensive molding procedures must be employed to provide a purely cylindrical bore. The use of expensive molding procedures of course significantly increases the cost of the overall assembly and the use of less costly molding procedures to produce a draft angle bore results in a bore that is not purely cylindrical with resultant piston binding and wear problems. Attempts have also been made to restore a purely cylindrical surface to a tapered bore utilizing a metallic sleeve made of steel, aluminum, or other metal or metal alloy and positioned within the tapered bore of the plastic cylinder body. However, this construction again adds complexity and expense to the cylinder assembly.
This invention is directed to the provision of a master cylinder assembly providing improved piston guidance.
More specifically, this invention is directed to a provision of a master cylinder assembly that is inexpensive to produce and yet provides excellent piston guidance.
The invention relates to a master cylinder of the type comprising a casing structure defining a bore and a piston mounted for sliding stroking movement in the bore between rearwardly retracted and forwardly extended positions and adapted for connection at a rearward end thereof to a piston rod. The casing structure further defines an annular groove in surrounding relation to the bore between the ends of the bore and further defines a reservoir port opening in the groove and communicating with a reservoir for holding hydraulic fluid.
According to the invention, the casing comprises a molded plastic structure; the bore includes a forward portion forwardly of the annular groove having a draft angle with the bore diameter forwardly decreasing; and the master cylinder further includes an annular bearing position at the forward end of the annular groove and having an internal cylindrical bearing surface for receiving and guiding the piston having a diameter approximating the diameter of the piston and greater than the diameter of the forward portion of the bore at the juncture of the forward bore portion with the forward end of the annular groove.
This arrangement allows inexpensive molding techniques to be utilized with respect to the forward portion of the bore while yet providing excellent bearing guidance for the piston.
According to a further feature of the invention, the bore further includes a rearward portion rearwardly of the annular groove defining a purely cylindrical bearing surface slidably receiving a rearward end of the piston. This arrangement allows expensive molding techniques to be utilized in association with the rearward end of the bore to produce a purely cylindrical bearing surface for coaction with the bearing member while continuing to allow inexpensive molding techniques to be utilized in association with the forward portion of the bore.
According to a further feature of the invention, the casing structure comprises two separate, molded plastic parts including a front part defining the forward draft angle bore portion and a rear port defining the cylindrical rearward bore portion. This arrangement clearly distinguishes between the relatively inexpensive technology required to produce the draft angle forward bore portion and the relatively expensive technology required to produce the cylindrical rearward bore portion.
According to a further feature of the invention, the master cylinder further includes an annular seal positioned in the annular groove forwardly of the port and rearwardly of the bearing member; the bearing member includes a main body annular portion seated against an annular shoulder defining the forward end of the annular groove and an annular lip portion extending rearwardly from the main body forward portion and having a crenellated configuration; and the seal includes a flexible annular lip portion positioned in annular surrounding relation to the bearing lip and movable in response to fluid pressure variations between an outwardly flexed position in which it blocks communication between the reservoir and the forward bore portion and an inwardly flexed position in which it allows communication between the reservoir and the forward bore portion through the bearing member lip crenelation. This arrangement allows the bearing member to further function as a routing device to allow recuperative fluid to flow into the forward portion of the bore behind the retreating piston.
According to a further feature of the invention, a plurality of circumferentially spaced axially extending and radially inwardly opening grooves are provided in the forward bore portion; and a plurality of circumferentially spaced axially extending and radially inwardly opening grooves are provided in the annular bearing face of the bearing member for coaction with the bore grooves, the bearing lip crenelation, and the seal lip. This arrangement allows communication between the forward bore portion and the reservoir even with the piston in the forwardly extended position received within the bearing member whereby fluid may readily flow from the reservoir to a location in the forward bore portion forwardly of the piston as the piston moves from a forwardly extended position to a rearwardly retracted position.
According to a further feature of the invention, the annular seal comprises a primary seal; and the master cylinder further includes a secondary annular seal positioned in the annular groove rearwardly of the reservoir port. This arrangement allows the use of primary and secondary seals to minimize leakage in the master cylinder assembly.
According to a further feature of the invention, the master cylinder further includes an annular spacer positioned in the annular groove between the primary seal and the secondary seal proximate the reservoir port and including passage means allowing the passage of fluid forwardly from the reservoir port to the primary seal. This arrangement allows the spacer to function to maintain proper spacing between the primary and secondary seals while yet allowing the flow of fluid forwardly from the reservoir port to the primary seal where it may flow through the crenelation of the bearing member and into the forward bore portion.
Other applications of the present invention will become apparent to those skilled in the art when the following description of the best mode contemplated for practicing the invention is read in conjunction with the accompanying drawings.